Organ transplantation requires the use of immunosuppressive drugs to prevent rejection of the transplanted organ, substantially limiting its benefit and augmenting its risk. Unfortunately, all regimens, regardless of the agents used, are incompletely and transiently effective, and induce some degree of immune incompetence for life. Since its discovery, costimulation blockade (CoB) has been suggested as a means of attaining indefinite, well-tolerated, antigen-specific prophylaxis from allograft rejection. An exemplary approach is blockade of the CD28/B7 pathway. Although exceedingly effective in some rodent models, CD28/B7 blockade is not alone sufficient to prevent rejection in primates, including humans. Several mechanisms of CoB- resistant rejection have been suggested, many related to the relative presence of activated alloreactive memory-phenotype T cells. We now have found that treatment with the CD2 specific fusion protein LFA-3-Ig (alefacept) synergizes with the CD28/B7-targeted fusion protein abatacept successfully preventing renal allograft rejection in primates. We hypothesize that alefacept specifically targets many of the factors fostering CoB resistance, including heterologous alloreactive T cell memory, while avoiding interference with peripheral mechanisms that serve to foster CoB-mediated allograft acceptance, including T cell regulation and donor antigen-specific T cell elimination. We therefore propose to investigate the mechanisms by which alefacept improved CoB and use this information to optimize its use, developing a clinically applicable therapy to prevent allograft rejection without the use of calcineurin inhibitors, steroids, or gross T cell depletion. In developing the regimen, we will specifically test alefacept's ability to eliminate pre-existing and induce memory T cells, evaluating the absolute and relative effects of alefacept on allospecific and memory T cells, and viral specific memory T cells. Throughout the study, we will determine the degree to which this regimen's influence on alloimmunity alters protective viral immunity. Based on preliminary evidence suggesting a salutary role for donor specific antigen infusion, we also will determine the degree to which antigen exposure at transplant improves the durability of the effect of this regimen, augmenting the allospecific effect without altering its effect on protective immunity. As this regimen uses agents that are approved for clinical use, this will provide data in support of an immediately translatable strategy to prevent rejection in humans, and establish generalizable knowledge regarding the role of T cell memory in thwarting CoB-based immune modulation. PUBLIC HEALTH RELEVANCE: Immunosuppressive drugs are required to prevent organ transplant rejection; unfortunately, an optimally effective, non-toxic drug regimen is yet to be developed. Costimulation blockade (CoB) has been shown to prevent rejection in rodents, but is far less effective in primates; however, we have developed a regimen pairing CoB with a fusion protein, alefacept, and have shown that it prevents renal allograft rejection in primates, hypothetically due to alefacept's effects countering CoB-resistant mechanisms of rejection. We propose to determine the mechanisms involved in alefacept's action focusing on its effects on allo- and viral-specific immunity, and in doing so develop this regimen into a well-tolerated, clinically applicable, and mechanistically sound therapy to prevent allograft rejection.